The rapid development and market growth of mobile devices and electric vehicles has resulted in strong demand for low cost, small size, lightweight, high energy density secondary batteries, such as lithium ion batteries. In the development of high energy density secondary batteries, cathode material technology is a well-recognized bottleneck, because cathode materials exhibit a lower capacity than anode materials. However cathode materials capacity improvement has been incremental, while anode capacity improvement can be improved many times by moving from conventional graphite to none carbon based anode materials. Consequently, there have been extensive development efforts to produce high capacity anode materials, because a higher capacity anode can significantly increase the energy density of a commercial secondary battery, for instance, by up to 25%, when the battery is fabricated using a given type of commercially available cathode material technology.
Silicon (Si) has been investigated as an anode material for lithium ion (Li+) batteries because it exhibits a high theoretical capacity (e.g., up to 3750-4200 mAh/g) as a battery cell anode material, and is an abundant, inexpensive element that is readily available as a result of its widespread use in the semiconductor industry. The electrochemical lithiation and delithiation of silicon can be generally represented asxLi++Si+xe−LixSi  (1)
With respect to the use of silicon as an anode material, this high theoretical capacity results in a significant theoretical increase in the energy density and specific energy of the cell compared to graphite anode materials.
Pure silicon anodes show excellent cyclic performance when a nano thin Si film is coated on conducting graphite/carbon, or when nano sized Si is composited with nano featured metal current collectors. Silicon nano wire and silicon nano particles also show good cyclic performance depending on the properties of various polymeric binders used therewith. However, these nano structured silicon anodes work well only at very low loading density. To increase the energy density of the battery cell, the loading density of the anode is increased. This means the anode is impregnated with a higher ratio of active material weight to inactive components in lithium ion cells. However, as the loading density increases, the electrodes collapse after initial cycles and the cyclic performance deteriorates.
Unfortunately, silicon anodes also exhibit a large first-cycle capacity loss, side reactions during cycling, and a very large volume change during battery cell charge-discharge or lithiation-delithiation cycles (e.g., up to 300-400%). With respect to this volume change, during lithium ion battery cycling, the Si anode is lithiated by intercalation (i.e., reversible insertion) of 4.4 Li atoms per Si atom. The very large volume change leads to mechanical failure and capacity fading.
Silicon oxide (SiOx) has also been investigated for use as an anode material for lithium ion batteries, particularly because this material shows much less volume change after the first cycle compared to pure silicon anode materials. SiOx is regarded as a uniform mixture of nano sized Si and SiO2 phases that form upon energetic treatment of original SiOx material, as described by K. Schulmeister and W. Mader in “TEM investigation on the structure of amporphous silicon monoxide,” Journal of Non-Crystalline Solids 320 (2003), pp. 143-150. When the molar ratio of Si to SiO2 is 1, its volumetric ratio is 0.5. This indicates that nano silicon particles are embedded in a matrix of SiO2 in the SiOx material structure.
The electrical conductivity of SiOx is low, and as SiOx is lithiated its electrical conductivity decreases. This poor electrical conductivity contributes to decreases in utilization of SiOx during cycling. The electrical conductivity of SiOx can be improved by mechanically milling SiOx (0.8<x<1.5) with graphite using high energy mechanical milling, as described in U.S. Pat. No. 6,638,662; or coating SiOx particles with a uniform carbon layer using thermal Chemical Vapor Deposition (CVD), as described in Japanese patent publication JP-A 2002-042806. These techniques successfully increase charge-discharge capacity, but fail to provide sufficient cyclic performance, and thus do not meet the market requirements for high energy density batteries. Therefore, such techniques have not been successfully utilized to produce commercial products in the market, as further improvement in cycle performance is imperative.
Another problematic electrochemical property of SiOx based anodes is a high irreversible capacity loss on the first charge/discharge cycle below a practical level, as described in U.S. Pat. No. 5,395,811. As indicated in U.S. Pat. No. 7,776,473, the irreversible capacity loss of SiOx anode material can be reduced by way of prelithiating the SiOx material.
U.S. Pat. Nos. 7,776,473 and 8,231,810 respectively indicate the following reactions between lithium and SiO:4Li+4SiO→Li4SiO4+3Si  (2)4Li+4SiO→3Si:Li4SiO4  (3)
The chemical reaction mainly forms lithium silicate (Li4SiO4) and silicon. In view of the aforementioned mixture of nano sized Si and SiO2 within SiOx, the reaction between lithium and SiOx can be expressed as follows:4Li+2SiO2→Li4SiO4+Si→Si:Li4SiO4(Si:LSC)  (4)
Depending on reaction conditions, some research groups have indicated that lithium silicate consists of Li4SiO4, Li2O, and Li2SiO3. The major component may be Li4SiO4. The irreversible chemical reaction of Li and the SiO2 matrix in the SiOx structure also forms a certain amount of lithium silicide (LiySi).
During the first lithiation of SiO, when the SiO2 phase irreversibly changes into the mixture of Li4SiO4 and LiySi, the volume increases by a factor of two. During delithiation, Li4SiO4 remains Li4SiO4, and LiySi becomes silicon. As a result, Si:LSC (Si:Li4SiO4) becomes porous, and because of plastic deformation of Li4SiO4, the volume change from LiySi:Li4SiO4 to Si:Li4SiO4 can be minimized. Consequently, the volume change of SiOx based anode particles is much smaller than that of pure silicon anode particles after the first cycle. Additionally, SiOx based anodes generally show much better cyclic performance compared with pure Si based anodes after the first cycle, under the condition that both are micro-sized.
U.S. Pat. No. 7,776,472 teaches the prelithiation of SiOx by milling SiOx powder with active lithium powder through a high energy ball milling process. This prelithiation successfully reduces the irreversible capacity loss from 35% to 15%. However, U.S. Pat. No. 7,776,472 also indicates that as a result of this prelithiation process, the reversible capacity was only 800 to 900 mAh/g, whereas most SiOx anodes coated with graphite show a reversible capacity of 1400 to 1700 mAh/g. Unfortunately, the results obtained by the process of U.S. Pat. No. 7,776,472 are not sufficient for satisfying the characteristics required for a commercial anode material. Lower irreversible capacity loss of the first cycle and improved cyclic performance are still required.
Further to the foregoing, while anode material prelithiation can reduce irreversible capacity loss, prelithiated anode materials have an undesirably high chemical reactivity due to the presence of highly reactive, chemically unstable lithium therein. This high chemical reactivity can lead to difficulties in handling and processing prelithiated anode materials during conventional battery manufacturing processes, or render the prelithiated anode materials incompatible with conventional battery manufacturing processes. For instance, prelithiated anode materials may be incompatible with solvents, binders, thermal processing conditions, and/or ambient environments commonly encountered in battery manufacturing processes. A need also exists to overcome this problem.